Power conversion device

ABSTRACT

A power conversion device, which includes an insulation type full bridge converter and can switch a power transmission direction at a high speed, is provided. A DC/DC converter ( 10 ) constitutes a power conversion device, which operates as a first type converter that converts a voltage within a first range applied to a first input/output terminal pair into a voltage within a second range and outputs the voltage from a second input/output terminal pair or a second type converter that converts a voltage within the second range applied to the second input/output terminal pair into a voltage within the first range and outputs the voltage from the first input/output terminal pair, as a device that performs a specific state transition of the DC/DC converter ( 10 ) after a change rate of a magnitude of a current flowing through a winding of a primary or secondary side of a transformer (TR) with respect to time reaches a value within a predetermined change rate range.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Japanese applicationserial no. 2016-093908, filed on May 9, 2016, and Japanese applicationserial no. 2017-045657, filed on Mar. 10, 2017. The entirety of each ofthe above-mentioned patent applications is hereby incorporated byreference herein and made a part of this specification.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a power conversion device, and moreparticularly to a power conversion device having a function ofconverting a voltage in a first range applied to a first input/outputterminal pair into a voltage in a second range and outputting thevoltage from a second input/output terminal pair and a function ofconverting a voltage in the second range applied to the secondinput/output terminal pair into a voltage in the first range andoutputting the voltage from the first input/output terminal pair.

Description of Related Art

As a bidirectional DC/DC converter in which large-capacity bidirectionalDC conversion is possible, an insulation type bidirectional DC/DCconverter (see, for example, Published Japanese Translation No.2015-517788 of the PCT International Publication) having a configurationin which two full bridge circuits are connected via a transformer isknown.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application PublicationNo. 2015-517788

SUMMARY OF THE INVENTION

If the insulation type bidirectional DC/DC converter (hereinafterreferred to as insulation type full bridge converter) having the aboveconfiguration performs a step-up operation or a step-down operation,ON/OFF of four switching elements in a full bridge circuit of a primaryside is controlled so that a state in which a direction of a currentflowing through a winding of the primary side of the transformer is afirst direction and a state in which the direction of the current is asecond direction opposite to the first direction alternate. Transitiontimings between the above-described two states are determinedirrespective of an amount (magnitude) of the current flowing through theprimary side/secondary side. Thus, if a power transmission directionfrom the insulation type full bridge converter is switched (ifinput/output terminals previously functioning as input terminalsfunction as output terminals), it is necessary to wait for a certainperiod of time for protection of an element in the converter or thelike.

The present invention has been made in view of the above-describedcircumstances, and an objective of the present invention is to provide apower conversion device which includes an insulation type full bridgeconverter and can switch a power transmission direction at a high speed(in a short time).

In order to achieve the above-described objective, a power conversiondevice of the present invention includes a first input/output terminalpair; a second input/output terminal pair; a DC/DC converter connectedto the first input/output terminal pair and the second input/outputterminal pair; and a control unit configured to control the DC/DCconverter. The DC/DC converter of the power conversion device of thepresent invention includes a first switching leg having first and secondswitching elements connected in series via a first connection point andconnected to the first input/output terminal pair; a second switchingleg having third and fourth switching elements connected in series via asecond connection point and connected in parallel to the first switchingleg; a third switching leg having fifth and seventh switching elementsconnected in series via a third connection point and connected to thesecond input/output terminal pair; a fourth switching leg having sixthand eighth switching elements connected in series via a fourthconnection point and connected in parallel to the third switching leg; afirst energy storage and conversion unit connected to the firstconnection point and the second connection point and connected to onewinding of a transformer and a first reactor connected in series; and asecond energy storage and conversion unit connected to the thirdconnection point and the fourth connection point and connected to theother winding of the transformer and a second reactor connected inseries.

Also, the control unit is able to execute a first control for causingthe DC/DC converter to convert a voltage within a first range applied tothe first input/output terminal pair into a voltage within a secondrange and to output the voltage within the second range from the secondinput/output terminal pair and a second control for causing the DC/DCconverter to convert a voltage within the second range applied to thesecond input/output terminal pair into a voltage within the first rangeand to output the voltage within the first range from the firstinput/output terminal pair. The first control capable of being executedby the control unit is a control for controlling ON/OFF of eachswitching element in the DC/DC converter so that a state of the DC/DCconverter iteratively transitions between, in an order of, a first statein which a current input from the first input/output terminal pair flowsthrough the first reactor, a second state in which a current is able tocirculate along a path including the first reactor, a third state inwhich the current input from the first input/output terminal pair flowsthrough the first reactor in a direction opposite to a direction in thefirst state, and a fourth state in which a current is able to circulatealong a path including the first reactor while flowing through the firstreactor in a direction opposite to a direction in the second state, andis a control for causing the state of the DC/DC converter to betransitioned from the second state to the third state after a first loadcurrent value, which is a value of a load current flowing through thefirst reactor, is within a first current value range if the first loadcurrent value is not within the first current value range when the stateof the DC/DC converter is to be transitioned from the second state tothe third state and causing the state of the DC/DC converter totransition from the fourth state to the first state after the first loadcurrent value is within the first current value range if the first loadcurrent value is not within the first current value range when the stateof the DC/DC converter is to be transitioned from the fourth state tothe first state. Also, the second control is a control for controllingON/OFF of each switching element in the DC/DC converter so that a stateof the DC/DC converter iteratively transitions between, in an order of,a fifth state in which a current input from the second input/outputterminal pair flows through the second reactor and no current is outputfrom the first input/output terminal pair, a sixth state in which thecurrent input from the second input/output terminal pair flows throughthe second reactor and a current is output from the first input/outputterminal pair, a seventh state in which no current is input from thesecond input/output terminal pair and no current is output from thefirst input/output terminal pair, an eighth state in which the currentinput from the second input/output terminal pair flows through thesecond reactor in a direction opposite to a direction in the fifth stateand no current is output from the first input/output terminal pair, aninth state in which the current input from the second input/outputterminal pair flows through the second reactor in a direction oppositeto a direction in the sixth state and a current is output from the firstinput/output terminal pair, and a tenth state in which no current isinput from the second input/output terminal pair and no current isoutput from the first input/output terminal pair and is a control forcausing the state of the DC/DC converter to transition from the sixthstate to the seventh state after a second load current value, which is avalue of a load current flowing through the second reactor, is within asecond current value range if the second load current value is notwithin the second current value range when the state of the DC/DCconverter is to be transitioned from the sixth state to the seventhstate and causing the state of the DC/DC converter to transition fromthe ninth state to the tenth state after the second load current valueis within the second current value range if the second load currentvalue is not within the second current value range when the state of theDC/DC converter is to be transitioned from the ninth state to the tenthstate. The power conversion device of the present invention has aconfiguration in which at least one of a determination of whether or notthe first load current value in the first control is within the firstcurrent value range and a determination of whether or not the secondload current value in the second control is within the second currentvalue range is performed on the basis of a pattern of change in a valueof a current flowing through the first reactor or the second reactorwith respect to time.

That is, for the DC/DC converter, it is effective to reduce a loadcurrent value at the time of transition from the second state to thethird state, a load current value at the time of transition from thefourth state to the first state, a load current value at the time oftransition from the sixth state to the seventh state, and a load currentvalue at the time of transition from the ninth state to the tenth stateso as to improve a speed of switching a power transmission direction ofthe power conversion device in which the first control for causing thestate of the DC/DC converter to iteratively transition between the firstto fourth states and the second control for causing the state of theDC/DC converter to iteratively transition between the fifth to tenthstates are performed. According to the power conversion device of thepresent invention having the above-described configuration, it ispossible to reduce the load current value at each transition time(reduce it to substantially “0”). Therefore, according to the powerconversion device of the present invention, the power transmissiondirection can be switched at a high speed (in a short time) as comparedwith a power conversion device in which the transition timing to variousstates is fixed.

The power conversion device of the present invention further includes acurrent sensor for measuring the value of the current flowing throughthe first reactor, wherein a configuration in which the control unitdetermines whether or not the first load current value is within thefirst current value range on the basis of the change pattern of thevalue of the current flowing through the first reactor measured by thecurrent sensor with respect to time during the first control, and thecontrol unit determines whether or not the second load current value iswithin the second current value range by using the value of the currentflowing through the first reactor measured by the current sensor as thesecond load current value during the second control may be adopted.

Also, the power conversion device of the present invention furtherincludes a current sensor for measuring the value of the current flowingthrough the second reactor, wherein a configuration in which the controlunit determines whether or not the second load current value is withinthe second current value range on the basis of the change pattern of thevalue of the current flowing through the second reactor measured by thecurrent sensor with respect to time during the second control, and thecontrol unit determines whether or not the first load current value iswithin the first current value range by using the value of the currentflowing through the second reactor measured by the current sensor as thefirst load current value during the first control may be adopted.

According to the present invention, it is possible to provide a powerconversion device which includes an insulation type full bridgeconverter and can switch a power transmission direction at a high speed(in a short time).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic configuration diagram of a power conversiondevice according to an embodiment of the present invention.

FIG. 1B is an explanatory diagram of a determination circuit provided ina control unit of the power conversion device according to theembodiment.

FIG. 2 is a timing chart of output control signals G1 to G4 when thecontrol unit provided in the power conversion device according to theembodiment causes a DC/DC converter to operate as a step-down converterwhose first input/output terminal side is a primary side.

FIG. 3 is a timing chart of output control signals G1 to G6 when thecontrol unit causes the DC/DC converter to operate as a step-upconverter whose first input/output terminal side is a primary side.

FIG. 4 is a time chart illustrating changes in currents flowing throughparts of the DC/DC converter with respect to time when the control unitcauses the DC/DC converter 10 to function as a step-down converter whosefirst input/output terminal side is the primary side together withchanges in the control signals G1 to G4 with respect to time.

FIG. 5A is an explanatory diagram illustrating current paths of aprimary side and a secondary side of the DC/DC converter when thecontrol unit causes the DC/DC converter to function as a step-downconverter whose first input/output terminal side is the primary side.

FIG. 5B is an explanatory diagram of the current paths of the primaryside and the secondary side of the DC/DC converter following FIG. 5A.

FIG. 5C is an explanatory diagram of the current paths of the primaryside and the secondary side of the DC/DC converter following FIG. 5B.

FIG. 5D is an explanatory diagram of the current paths of the primaryside and the secondary side of the DC/DC converter following FIG. 5C.

FIG. 5E is an explanatory diagram of the current paths of the primaryside and the secondary side of the DC/DC converter following FIG. 5D.

FIG. 5F is an explanatory diagram of the current paths of the primaryside and the secondary side of the DC/DC converter following FIG. 5E.

FIG. 6 is a flowchart of a step-down control process executed by thecontrol unit.

FIG. 7 is a diagram illustrating the significance of the processing ofsteps S104 and S109 of the step-up control process.

FIG. 8 is a flowchart of a step-up control processing executed by thecontrol unit.

FIG. 9 is a time chart illustrating changes in currents flowing throughparts of the DC/DC converter with respect to time when the control unitcauses the DC/DC converter to function as a step-up converter whosefirst input/output terminal side is the primary side together withchanges in control signals G1 to G6 with respect to time.

FIG. 10A is an explanatory diagram of current paths of a primary sideand a secondary side of a DC/DC converter when the control unit causesthe DC/DC converter to function as a step-up converter whose firstinput/output terminal side is the primary side.

FIG. 10B is an explanatory diagram of current paths of the primary sideand the secondary side of the DC/DC converter following FIG. 10A.

FIG. 10C is an explanatory diagram of the current paths of the primaryside and the secondary side of the DC/DC converter following FIG. 10B.

FIG. 10D is an explanatory diagram of the current paths of the primaryside and the secondary side of the DC/DC converter following FIG. 10C.

FIG. 10E is an explanatory diagram of the current paths of the primaryside and the secondary side of the DC/DC converter following FIG. 10D.

FIG. 10F is an explanatory diagram of the current paths of the primaryside and the secondary side of the DC/DC converter following FIG. 10E.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

FIG. 1A is a schematic configuration diagram of a power conversiondevice according to an embodiment of the present invention.

The power conversion device according to the present embodiment is adevice in which bidirectional power conversion is possible. Asillustrated, the power conversion device includes a DC/DC converter 10,a control unit 20, and two pairs of input/output terminals 13 (13 p and13 m). In any pair of input/output terminals 13, the input/outputterminal 13 p is an input/output terminal of a high potential side andthe input/output terminal 13 m is an input/output terminal of a lowpotential side.

The DC/DC converter 10 is an insulation type bidirectional DC/DCconverter including a transformer TR, two reactors Lr, and two fullbridge circuits 11 as main components. Hereinafter, the full bridgecircuit 11 on the left side and the full bridge circuit 11 on the rightside in FIG. 1A are referred to as a first full bridge circuit 11 and asecond full bridge circuit 11, respectively. Likewise, the reactors Lron the left side and the right side in FIG. 1A are referred to as afirst reactor and a second reactor, respectively, and a winding on theleft side and a winding on the right side in the transformer TR in FIG.1A are referred to as a first winding and a second winding,respectively. Also, the input/output terminals 13 (13 p and 13 m) on theleft side and the right side in FIG. 1A are referred to as a firstinput/output terminal 13 and a second input/output terminal 13,respectively. Current sensors 15 on the left side and the right side inFIG. 1A are referred to as a first current sensor 15 and a secondcurrent sensor 15, respectively. The transformer TR of the DC/DCconverter 10 may not have a turn ratio of 1:1. However, theconfiguration and operation of the power conversion device will bedescribed below under the assumption that the turn ratio of thetransformer TR is 1:1.

The first full bridge circuit 11 of the DC/DC converter 10 includes afirst switching leg having a first switching element SW1 and a secondswitching element SW2 connected in series and a second switching leghaving a third switching element SW3 and a fourth switching element SW4connected in series. As illustrated, a diode Dn is connected in parallelbetween terminals of an n^(th) switching element SWn (n=1 to 4) of eachswitching leg. Also, each switching leg is connected to a pair of firstinput/output terminals 13 and a connection point between the first andsecond switching elements SW1 and SW2 of the first switching leg isconnected to one end of the first winding of the transformer TR via thefirst reactor. A connection point between the third and fourth switchingelements SW3 and SW4 of the second switching leg is connected to theother end of the first winding of the transformer TR.

The second full bridge circuit 11 of the DC/DC converter 10 includes athird switching leg having a fifth switching element SW5 and a seventhswitching element SW7 connected in series and a fourth switching leghaving a sixth switching element SW6 and an eighth switching element SW8connected in series. As illustrated, a diode Dn is connected in parallelbetween terminals of an n^(th) switching element SWn (n=5 to 8) of eachswitching leg. Both the third switching leg and the fourth switching legare connected to the second input/output terminal pair. A connectionpoint between the fifth and seventh switching elements SW5 and SW7 ofthe third switching leg is connected to one end of the second winding ofthe transformer TR via the second reactor, and a connection pointbetween the sixth and eighth switching elements SW6 and SW8 of thefourth switching leg is connected to the other end of the second windingof the transformer TR.

Two current sensors 15 for measuring the magnitude of the currentflowing through each reactor Lr are attached to the DC/DC converter 10.Various sensors (not illustrated) for measuring magnitudes of aninput/output voltage and an input/output current are also attached tothe DC/DC converter 10.

The control unit 20 is a unit that controls the DC/DC converter 10(ON/OFF of each switching element in the DC/DC converter 10) by changingthe level of the control signal for each switching element in the DC/DCconverter 10. Hereinafter, a control signal for the n^(th) switchingelement SWn (n=1 to 8) is referred to as a control signal Gn.

The control unit 20 is a unit for controlling the DC/DC converter 10.The control unit 20 includes a processor (a microcontroller or thelike), a gate driver, two determination circuits 22, and the like, andan output of a first current sensor 15 is input to one determinationcircuit 22 (hereinafter referred to as a first determination circuit22). Also, an output of a second current sensor 15 is input to the otherdetermination circuit 22 (hereinafter referred to as a seconddetermination circuit 22).

In FIG. 1B, a configuration of the determination circuit 22 isillustrated. As illustrated, the determination circuit 22 includes a lowpass filter (LPF) 25 for removing a low frequency component from anoutput of the current sensor 15, a differential circuit 26 thatdifferentiates an output of the LPF 25, and a window comparator 27 thatoutputs a determination result of whether or not an output of thedifferential circuit 26 is a value within a range from −Vth to Vth. Afield of application of each determination circuit 22 and referencevoltages −Vth and Vth input to the window comparator 27 of eachdetermination circuit 22 will be described below.

Returning to FIG. 1A, description of the control unit 20 will becontinued.

The control unit 20 determines whether to operate the DC/DC converter 10as one of converters of the following four types on the basis of inputdata (a current value and a voltage value) and is configured(programmed) to control the DC/DC converter 10 so that the DC/DCconverter operates as the determined converter.

-   -   A step-up converter whose first input/output terminal 13 side is        the primary side.    -   A step-down converter whose first input/output terminal 13 side        is the primary side.    -   A step-up converter whose second input/output terminal 13 side        is the primary side.    -   A step-down converter whose second input/output terminal 13 side        is the primary side.

Also, the control unit 20 is configured (programmed) so that a change incontrol details for the DC/DC converter 10 (a change from the controlfor causing the DC/DC converter 10 to operate as the step-up converterwhose first input/output terminal 13 side is the primary side to thecontrol for causing the DC/DC converter 10 to operate as the step-downconverter whose second input/output terminal 13 side is the primary sideor the like) is immediately performed.

Hereinafter, the configuration and operation of the power conversiondevice according to the present embodiment will be specificallydescribed.

First, details of basic control of the DC/DC converter 10 by the controlunit 20 will be described. Details of the control performed on the firstand second full bridge circuits 11 by the control unit 20 when the DC/DCconverter 10 is operated as the step-up/step-down converter whose secondinput/output terminal 13 side is the primary side are the same asdetails of the control performed on the second and first full bridgecircuits 11 by the control unit 20 when the DC/DC converter 10 isoperated as the step-up/step-down converter whose first input/outputterminal 13 side is the primary side. In other words, the controldetails of the control unit 20 when the DC/DC converter 10 is operatedas the step-up/step-down converter whose second input/output terminal 13side is the primary side are substantially the same as the controldetails of the control unit 20 when the DC/DC converter 10 is operatedas the step-up/step-down converter whose first input/output terminal 13side is the primary side. Thus, only the control details of the controlunit 20 when the DC/DC converter 10 is operated as the step-up/step-downconverter whose first input/output terminal 13 side is the primary sidewill be described.

Also, control signals G1 to G8 actually output by the control unit 20are configured so that ON and OFF of the two switching elements (ON ofthe first switching element SW1, OFF of the second switching elementSW2, and the like) are performed with a time difference (so-called deadtime). However, the operation of the control unit 20 will be describedbelow under the assumption that the time difference is not assigned toavoid complicated explanation.

(1) When the DC/DC converter 10 functions as a step-down converter whosefirst input/output terminal 13 side is the primary side:

In this case, the control unit 20 basically outputs control signals G1to G4 which change with time as illustrated in FIG. 2.

In other words, when the DC/DC converter 10 functions as the step-downconverter whose first input/output terminal 13 side is the primary side,the control unit 20 outputs the control signal G1 whose ON time is 1/2of a period T, i.e., the control signal G1 having a duty ratio of 1/2.Also, the control unit 20 outputs the control signal G2 obtained byinverting the control signal G1, i.e., the control signal G2 by whichthe second switching element SW2 is turned OFF/ON when the firstswitching element SW1 is ON/OFF. Further, the control unit 20 outputsthe control signal G3 whose phase is shifted by θ from the controlsignal G1 and the control signal G4 obtained by inverting the controlsignal G3.

Then, while the control signals G1 to G4 as described above are output,the control unit 20 changes the value of 0 so that the output voltage orthe output current of the DC/DC converter 10 become a target value.

Also, when the power conversion device according to the presentembodiment operates as the step-down device whose first input/outputterminal 13 side is the primary side, the second full bridge circuit 11(the full bridge circuit 11 of the secondary side; see FIG. 1A) is usedas a full wave rectifier (diode bridge circuit). Accordingly, when theDC/DC converter 10 is operated as the step-down converter whose firstinput/output terminal 13 side is the primary side, the control unit 20maintains the state of each switching element in the second full bridgecircuit 11 in an OFF state.

(2) When the DC/DC converter 10 functions as a step-up converter whosefirst input/output terminal 13 side is the primary side: In this case,the control unit 20 basically outputs the control signals G1 to G6 whichchange with time as illustrated in FIG. 3.

In other words, when the DC/DC converter 10 functions as the step-upconverter whose first input/output terminal 13 side is the primary side,the control unit 20 outputs the control signal G2 having a duty ratio of1/2 and the control signal G1 which rises at a falling edge time of thecontrol signal G2 and has a shorter ON time than the control signal G2.Also, the control unit 20 outputs the control signals G3 and G4, whichare signals obtained by delaying the control signals G1 and G2 by T/2,respectively. Further, the control unit 20 outputs the control signal G5having a high level only during a time Ton from the falling edge time ofthe control signal G1 and the control signal G6 having a high level onlyduring the time Ton from the falling edge time of the control signal G3.

Then, while the control signals G1 to G6 as described above are output,the control unit changes a value of the time Ton so that the outputvoltage or the output current of the DC/DC converter 10 becomes thetarget value.

Hereinafter, details of control of the DC/DC converter 10 by the controlunit 20 will be more specifically described.

First, an operation of the control unit 20 when the DC/DC converter 10functions as a step-down converter whose first input/output terminal 13side is the primary side will be described.

FIG. 4 illustrates changes in currents flowing through parts of theDC/DC converter 10 with respect to time when the control unit 20 causesthe DC/DC converter 10 to function as the step-down converter whosefirst input/output terminal 13 side is the primary side together withchanges in the control signals G1 to G4 with respect to time. Also,FIGS. 5A to 5F are explanatory diagrams illustrating current paths of aprimary side and a secondary side of the DC/DC converter 10 when thecontrol unit 20 causes the DC/DC converter 10 to function as thestep-down converter whose first input/output terminal 13 side is theprimary side. Also, in FIG. 4 and FIG. 9 described below, an “inputcurrent” and an “output current” are a current flowing into the firstinput/output terminal 13 p and a current flowing out from the secondinput/output terminal 13 p, respectively. Also, an “input side Lrcurrent” and an “output side Lr current” are a current flowing throughthe first reactor (the reactor Lr of the primary side) and a currentflowing through the second reactor (the reactor Lr of the secondaryside).

As described above, when the DC/DC converter 10 is operated as thestep-down converter whose first input/output terminal 13 side is theprimary side, the control unit 20 outputs the control signals G1 to G4that change with time as illustrated in FIG. 4.

Accordingly, when the control unit 20 operates the DC/DC converter 10 asthe step-down converter whose first input/output terminal 13 side is theprimary side, the state of the DC/DC converter 10 iterativelytransitions in the order of state #1, state #2, state #3, and state #4between the following four states.

-   -   State #1: a state in which the first switching element SW1 and        the fourth switching element SW4 are ON (the first switching        element SW1 and the fourth switching element SW4 are ON and the        other switching elements are OFF; this is the same in the        following).    -   State #2: a state in which the second switching element SW2 and        the fourth switching element SW4 are ON.    -   State #3: a state in which the second switching element SW2 and        the third switching element SW3 are ON.    -   State #4: a state in which the first switching element SW1 and        the third switching element SW3 are ON.

When the state of the DC/DC converter 10 is in state #1, as illustratedin FIG. 5A, a current flows through a path of first input/outputterminal 13 p→first switching element SW1→first reactor→transformerTR→fourth switching element SW4→first input/output terminal 13 m.Accordingly, energy is stored in the first reactor and the input side Lrcurrent rises as illustrated in FIG. 4. Also, energy is transmitted tothe secondary side via the transformer TR and rectified by the secondfull bridge circuit 11 and output from the second input/output terminal13 as illustrated in FIGS. 4 and 5A.

When the state of the DC/DC converter 10 transitions to state #2, thefirst switching element SW1 is OFF and the second switching element SW2is ON. Accordingly, as illustrated in FIG. 5B, a current circulatesalong a path of second switching element SW2 (and diode D2)→firstreactor→transformer TR→fourth switching element SW4 according to energystored in the first reactor. As a result, the energy stored in the firstreactor is transmitted to the secondary side, rectified by the secondfull bridge circuit 11, and output from the second input/output terminal13.

The magnitude of the circulating current in state #2 decreases with themovement of the energy stored in the first reactor to the secondaryside. Therefore, when a certain time (time corresponding to the θ value)has elapsed after the transition to state #2, as illustrated in FIG. 5C,a state where no current flows through the second reactor is formed.Also, when a magnitude of a load current of the secondary side of thetransformer TR (i.e., a current flowing through the second reactor) is“0”, a magnitude of a load current of the primary side of thetransformer TR is also “0.” Because the current flowing through theprimary side of the transformer TR is the load current+the excitationcurrent, a current circulating through the primary side of the DC/DCconverter 10 is the excitation current in FIG. 5C.

The DC/DC converter 10 is configured so that the state is a stateillustrated in FIG. 5C in principle (if a load is not excessively large)when the original timing of transition to state #3 has been reached.Here, the original transition timing is a transition timing when atiming adjustment function (of which details will be described below)provided in the control unit 20 is not working.

When the state of the DC/DC converter 10 is state #3 (i.e., when SW2 andSW3 are ON), as illustrated in FIG. 5D, a current flows through a pathof first input/output terminal 13 p→third switching elementSW3→transformer TR→first reactor→second switching element SW2→firstinput/output terminal 13 m. In other words, a current flows through thefirst reactor in a direction opposite to that in a case in which thestate of the DC/DC converter 10 is state #1 (FIG. 5A).

Accordingly, energy is stored in the first reactor (see FIG. 4) andenergy is transmitted to the secondary side via the transformer TR,rectified by the second full bridge circuit 11, and output from thesecond input/output terminal 13.

When the primary side circuit of the DC/DC converter 10 transitions fromstate #3 to state #4, as illustrated in FIG. 5E, a current circulatesalong a path of third switching element SW3→transformer TR→firstreactor→first switching element SW1 (and diode D1) according to theenergy stored in the first reactor. Thus, the energy stored in the firstreactor is transmitted to the secondary side.

The magnitude of the circulating current in this state #4 also decreaseswith movement of energy stored in the first reactor to the secondaryside. When a certain time has elapsed after the transition to state #4,a state in which no current flows through the second reactor, i.e., astate in which load currents of the primary side and the secondary sidedo not flow, is formed as illustrated in FIG. 5F. Even when the originaltiming of transition from state #4 to state #1 has been reached, theDC/DC converter 10 is configured to make the state be a stateillustrated in FIG. 5F in principle.

As described above, when the original timing of transition from thestate (state #2 or state #4) in which the current circulates in thecircuit on the primary side to a next state has been reached, the DC/DCconverter 10 is configured so that the magnitude of the second reactorcurrent becomes “0.” However, it is difficult to configure the DC/DCconverter 10 so that the above-described condition is satisfied withoutdegrading the performance of the power conversion device in alloperation conditions of the power conversion device.

Thus, when the control unit 20 (the microcontroller in the control unit20) causes the DC/DC converter 10 to operate as the step-down converterwhose first input/output terminal 13 side is the primary side, thecontrol unit 20 is configured (programmed) so as to perform thestep-down control process of a procedure illustrated in FIG. 6.

That is, the control unit 20 that started the step-down control processfirst performs a timer setting process of setting a specified value inan interrupt timer in step S100. Here, the interrupt timer is a timerthat generates an interruption when a time according to a set value haselapsed. Also, a specified value is a value at which the interrupt timerwill generate an interruption after the passage of a time T/2 (T is aperiod of the control signal: see FIG. 2) if the value is set.

Further, in step S100, the control unit 20 also performs a process ofturning the first switching element SW1 and the fourth switching elementSW4 OFF by changing the levels of the control signals G1 and G4 to ahigh level.

Upon completion of the processing of step S100, the control unit 20waits (monitors) an SW1 and SW2 control timing in step S101. Here, theSW1 and SW2 control timing is a timing determined by a θ value tocontrol (change) the ON/OFF states of the first switching element SW1and the second switching element SW2. The processing of step S101 to befirst executed after the start of the step-down control process may beperformed irrespective of the θ value (e.g., a process of determiningthat the SW1 and SW2 control timing has been reached when apredetermined time has elapsed from a time point at which step S100 wasexecuted). Also, the processing of step S101 of the second andsubsequent times may be a process of determining whether or not the SW1and SW2 control timing has been reached on the basis of an elapsed timeafter the occurrence of an interruption or a process of determiningwhether or not the SW1 and SW2 control timing has been reached on thebasis of an elapsed time after the first switching element SW1 waspreviously turned ON.

When the SW1 and SW2 control timing has been reached (step S101; YES),the control unit 20 changes the levels of the control signals G1 and G2to turn the first switching element SW1 OFF and turn the secondswitching element SW2 ON (step S102). In this step S102, the controlunit 20 assigns a time difference to ON/OFF of the first switchingelement SW1 and the second switching element SW2 to suppress athrough-current. More specifically, the control unit 20 changes thelevel of the control signal G2 when a predetermined time has elapsedafter a change in the level of the control signal G1.

Thereafter, the control unit 20 waits (monitors) the occurrence of aninterruption in step S103. When the interruption has occurred (stepS103; YES), the second unit 20 determines whether or not a first changerate, which is a change rate in a value of a current flowing through thefirst reactor with respect to time, is within a first change rate range(step S104). Here, the first change rate range is a determined changerate range centered on “0” so that a power transmission direction can beswitched at a desired speed if the processing of steps S105 and S110 (ofwhich details will be described below) is performed when the firstchange rate is within the range.

During the step-down control process, the window comparator 27 of thefirst determination circuit 22 receives an output voltage of thedifferential circuit 26 when the first change rate is a lower limitvalue of the first change rate range and an output voltage of thedifferential circuit 26 when the first change rate is an upper limitvalue of the first change rate range as the reference voltages −Vth andVth (see FIG. 1B). That is, the first determination circuit 22 functionsas a circuit that outputs a signal indicating whether or not the firstchange rate is within the first change rate range during the step-downcontrol process. In step S104, the control unit 20 determines whether ornot the first change rate is within the first change rate range on thebasis of a signal from the first determination circuit 22.

If the first change rate is within the first change rate range (stepS104; YES), the control unit 20 sets a specified value in the interrupttimer (step S105). Also, the control unit 20 turns the fourth switchingelement SW4 OFF and turns the third switching element SW3 ON by changingthe levels of the control signals G3 and G4 (step S105). Also, if thefirst change rate is not within the first change rate range (step S104;NO), the control unit 20 waits for the first change rate to be a valuewithin the first change rate range by iterating the processing of stepS104, and then performs the processing of S105.

Also, in step S105 and steps S107 and S110 to be described later,similar to the execution of the above-described step S102, the controlunit 20 assigns a time difference to ON/OFF of the two switchingelements to suppress a through-current.

Upon completion of the processing of step S105, the control unit 20waits (monitors) the SW1 and SW2 control timing (step S106). The SW1 andSW2 control timing in this step S106 is also a timing determined by theθ value and a Ton value to control (change) the ON/OFF states of thefirst switching element SW1 and the second switching element SW2. Theprocessing of step S106 may be a process of determining whether or notthe SW1 and SW2 control timing has been reached on the basis of thepassage of time after the first switching element SW1 was previouslyturned OFF or a process of determining whether or not the SW1 and SW2control timing has been reached on the basis of the passage of timeafter the occurrence of an interruption.

When the SW1 and SW2 control timing has been reached (step S106; YES),the control unit 20 changes the levels of the control signals G1 and G2to turn the first switching element SW1 ON and turn the second switchingelement SW2 OFF (step S107).

Thereafter, the control unit 20 waits for occurrence of an interruptionin step S108. If the interruption has occurred (step S108; YES), thecontrol unit 20 determines whether or not the first change rate at aninterruption time is within the first change rate range on the basis ofa signal from the first determination circuit 22 (step S109).

If the first change rate is within the first change rate range (stepS109; YES), the control unit 20 sets a specified value in the interrupttimer, turns the third switching element SW3 OFF, and turns the fourthswitching element SW4 ON (step S110). Also, if the first change rate isnot within the first change rate range (step S109; NO), the control unit20 waits for the first change rate to be within the first change raterange, and then performs the processing of step S110.

Then, the control unit 20 that has completed the processing of step S110resumes step S101 and the processing subsequent thereto.

Hereinafter, the significance of the processing of steps S104 and S109in the step-down control process (FIG. 6) will be described.

To switch the power transmission direction at a high speed by the DC/DCconverter 10, it is only necessary to perform the processing of stepsS105 and S110 after a load current value of the primary side of thetransformer TR becomes small during the step-down control process.However, a large excitation current according to an input voltage flowsthrough the primary side of the transformer TR. Because a current valuemeasured by the first current sensor 15 is (excitation currentvalue+load current value), a state in which the load current valuedecreases to a desired value may not be detected by merely comparing acurrent value measured by the first current sensor 15 with a thresholdvalue.

In steps S104 and S109, it is determined whether or not the first changerate is within the first change rate range to prevent such a malfunctionfrom occurring. Specifically, a current value measured by the firstcurrent sensor 15 changes with time as illustrated in FIG. 7, but achange rate of an excitation current value is significantly low. Thus,if a load current component is not included in a result of measuring acurrent value from the first current sensor 15, a change rate of thecurrent value with respect to time is substantially “0” as illustratedin FIG. 7. Accordingly, it is possible to determine whether or not theload current value of the primary side of the transformer TR issignificantly small (substantially “0”) according to whether or not thefirst change rate is within the first change rate range.

Next, an operation of the control unit 20 when the DC/DC converter 10functions as the step-up converter whose first input/output terminal 13side is the primary side will be described.

If the DC/DC converter 10 is operated as the step-up converter whosefirst input/output terminal 13 side is the primary side, the controlunit 20 is configured (programmed) to perform the step-up controlprocess of the procedure illustrated in FIG. 8.

In other words, the control unit 20 that started the step-up controlprocess firstly sets a specified value in the interrupt timer (stepS200). Also, in step S200, the control unit 20 performs a process ofturning the first switching element SW1, the fourth switching elementSW4, and the fifth switching element SW5 ON by changing the levels ofthe control signals G1, G4, and G5 to a high level.

The control unit 20 that has completed the processing of step S200 waits(monitors) an SW5 control timing in step S201. Here, the SW5 controltiming is a timing determined by the Ton value and at which the fifthswitching element SW5 is to be turned OFF such that it is determined bythe Ton value.

When the SW5 control timing has been reached (step S201; YES), thecontrol unit 20 turns the fifth switching element SW5 OFF by changingthe level of the control signal G5 (step S202).

Thereafter, the control unit 20 determines whether or not the firstchange rate is within a second change rate range (step S203). Here, thesecond change rate range is a determined change rate range centered on“0” so that the power transmission direction can be switched at adesired speed if the processing of steps S204 and S212 is performed whenthe first change rate is within the range.

During the step-up control process, the window comparator 25 of thefirst determination circuit 22 receives an output voltage of thedifferential circuit 26 when the first change rate is the lower limitvalue of the second change rate range and an output voltage of thedifferential circuit 26 when the first change rate is the upper limitvalue of the second change rate range as the reference voltage −Vth andVth (see FIG. 1B). In other words, the first determination circuit 22functions as a circuit that outputs a signal indicating whether or notthe first change rate is within the second change rate range during thestep-up control process. In step S203, the control unit 20 determineswhether or not the first change rate is within the second change raterange on the basis of a signal from the first determination circuit 22.

If the first change rate is not within the second change rate range(step S203; NO), the control unit 20 iterates the processing(determination) of step S203. If the first change rate is within thesecond change rate range (step S203; YES), the control unit 20 turns thefirst switching element SW1 OFF (step S204). Next, the control unit 20waits for occurrence of an interruption (step S205). If the interruptionhas occurred (step S205; YES), the control unit 20 sets a specifiedvalue in the interruption timer, turns the fourth switching element SW4OFF, and turns the second switching element SW2, the third switchingelement SW3, and the sixth switching element SW6 ON (step S206). Also,if the interruption has already occurred during the execution of stepS203 (or step S204), the control unit 20 performs the processing of stepS206 without waiting for the occurrence of the interruption in stepS205.

The control unit 20 that has completed the processing of step S206awaits (monitors) an SW6 control timing in step S207. Here, the SW6control timing is a timing determined by the Ton value and at which thesixth switching element SW6 is to be turned OFF such that it isdetermined by the Ton value.

When the SW6 control timing has been reached (step S207; YES), thecontrol unit 20 turns the sixth switching element SW6 OFF by changingthe level of the control signal G6 (step S208).

Thereafter, the control unit 20 determines whether or not the firstchange rate is within the second change rate range (step S209). Theprocessing of step S209 is the same as the processing of step S204.

If the first change rate is not within the second change rate range(step S209; NO), the control unit 20 iterates the processing(determination) of step S209. If the first change rate is within thesecond change rate range (step S209; YES), the control unit 20 turns thethird switching element SW3 OFF (step S210). Next, the control unit 20waits for occurrence of an interruption (step S211). If the interruptionhas occurred (step S211; YES), the control unit 20 sets a specifiedvalue in the interruption timer, turns the second switching element SW2OFF, and turns the first switching element SW1, the fourth switchingelement SW4, and the fifth switching element SW5 ON (step S212). If theinterruption has already occurred during the execution of step S209 (orstep S210), the control unit 20 performs the processing of step S212without waiting for the occurrence of the interruption in step S211.

Then, the control unit 20 that has completed the processing of step S212resumes a process of moving to step S201.

Hereinafter, details of the step-up control process will be morespecifically described.

FIG. 9 illustrates changes in currents flowing through parts of theDC/DC converter 10 with respect to time when the control unit 20 causesthe DC/DC converter 10 to function as a step-up converter whose firstinput/output terminal 13 side is the primary side together with changesin control signals G1 to G6 with respect to time. Also, FIGS. 10A to 10Fare explanatory diagrams of current paths of a primary side and asecondary side of the DC/DC converter 10 when the control unit 20 causesthe DC/DC converter 10 to function as the step-up converter whose firstinput/output terminal 13 side is the primary side.

When the step-up control process of the above-described details isperformed, the state of the DC/DC converter 10 iteratively transitionsbetween the following six states in the order of state #5, state #6,state #7, state #8, state #9, and state #10 (see FIG. 9).

-   -   State #5: a state in which the first switching element SW1, the        fourth switching element SW4, and the fifth switching element        SW5 are ON.    -   State #6: a state in which the first switching element SW1 and        the fourth switching element SW4 are ON.    -   State #7: a state in which the fourth switching element SW4 is        ON.    -   State #8: a state in which the second switching element SW2, the        third switching element SW3, and the sixth switching element SW6        are ON.    -   State #9: a state in which the second switching element SW2 and        the third switching element SW3 are ON.    -   State #10: a state in which the second switching element SW2 is        ON.

When the state of the DC/DC converter 10 is state #5 (i.e., when SW1,SW4, and SW5 are ON), as illustrated in FIG. 10A, a current flowsthrough a path of first input/output terminal 13 p→first switchingelement SW1→first reactor→transformer TR→fourth switching elementSW4→first input/output terminal 13 m. Accordingly, energy is stored inthe first reactor and the input side Lr current rises as illustrated inFIG. 9. Also, energy is transmitted to the secondary side via thetransformer TR. However, because the fifth switching element SW5 of thesecond full bridge circuit 11 is ON, as illustrated in FIG. 10A, acurrent circulates along a path of diode D6→transformer TR→secondreactor→fifth switching element SW5 on the secondary side of the DC/DCconverter 10. Accordingly, energy from the primary side is not outputfrom the second input/output terminal 13, but stored in the secondreactor.

When the fifth switching element SW5 is OFF and the state of the DC/DCconverter 10 becomes state #6, as illustrated in FIG. 10B, the energystored in the first reactor and energy from a power supply aretransmitted to the secondary side and the current rectified by thesecond full bridge circuit 11 is output from the second input/outputterminal 13.

Because the output voltage of the load is applied to the secondinput/output terminal 13, the input current, the input side Lr current,and the output current gradually decrease if the state of the DC/DCconverter 10 is state #6. Then, when it is detected that the firstchange rate is within the first change rate range in step S203 (FIG. 7),the first switching element SW1 is OFF and the state of the DC/DCconverter 10 transitions to state #7. As a result, as illustrated inFIG. 10C, a state in which no load current flows through the primaryside and the secondary side of the DC/DC converter 10 is formed. As thestate illustrated in FIG. 10C, a state in which no load current flowsthrough the primary side and the secondary side of the DC/DC converter10 is formed.

When the state of the DC/DC converter 10 transitions from state #7 tostate #8, as illustrated in FIG. 10D, a current flows through a path offirst input/output terminal 13 p→third switching element SW3→transformerTR→first reactor→second switching element SW2→first input/outputterminal 13 m. Accordingly, energy is stored in the first reactor (seeFIG. 9), and energy is transmitted to the secondary side via thetransformer TR. However, because the sixth switching element SW6 is ON,as illustrated in FIG. 10D, a current circulates along a path of diodeD5→second reactor→transformer TR→sixth switching element SW6 on thesecondary side of the DC/DC converter 10. Accordingly, in state #8, theenergy from the primary side is stored in the second reactor.

If the sixth switching element SW6 is OFF and the state of the DC/DCconverter 10 is shifted to state #9, energy stored in the second reactorand energy from the power supply are transmitted to the secondary sideand a current rectified by the second full bridge circuit 11 is outputfrom the second input/output terminal 13 as illustrated in FIG. 10E.

An output voltage of a load is applied to the second input/outputterminal 13. Thus, the input current, the input Lr current, and theoutput current in state #9 gradually decrease. Then, if the second Lrcurrent value “0” is detected in step S209 (FIG. 8), the third switchingelement SW3 is turned OFF to be in state #10. Accordingly, when thestate of the DC/DC converter 10 is state #10, as illustrated in FIG.10F, a state in which no load current flows through the primary side andthe secondary side of the DC/DC converter 10 is formed.

As described above, in the power conversion device according to thepresent embodiment, the transition from the second state to the thirdstate and the transition from the fourth state to the first state areperfonned for the step-down time when the first change rate has reacheda value within the first change rate range (the load current value ofthe primary side has reached substantially “0”). Also, the transitionfrom the sixth state to the seventh state and the transition from theninth state to the tenth state are performed for the step-up time whenthe first change rate has reached a value within the second change raterange (the load current value of the primary side has reached “0”). Forthe DC/DC converter 10, it is effective to reduce a load current valueat the time of transition from the second state to the third state, aload current value at the time of transition from the fourth state tothe first state, a load current value at the time of transition from thesixth state to the seventh state, and a load current value at the timeof transition from the ninth state to the tenth state so as to improve aspeed of switching a power transmission direction of the powerconversion device in which a control for causing the state of the DC/DCconverter 10 to iteratively transition between the first to fourthstates and control for causing the state of the DC/DC converter 10 toiteratively transition between the fifth to tenth states are performed.Accordingly, the power conversion device of the present invention canswitch the power transmission direction at a higher speed (in a shortertime) than a conventional power conversion device.

Modified Embodiments

The power conversion device according to the above-described embodimentcan have various modifications. For example, the power conversion devicecan be modified to a device which does not include the determinationcircuit 22 and in which it is determined whether or not the first changerate has reached a value within the first/second change rate range insoftware. Also, the second determination circuit 22 outputs the samesignal as a signal output by the first determination circuit 22 inprinciple. Accordingly, if the second current sensor 15 and the seconddetermination circuit 22 are removed and the second input/outputterminal 13 is the primary side, the timing of transition to variousstates may be determined on the basis of the output of the firstdetermination circuit 22. Of course, if the first current sensor 15 andthe first determination circuit 22 are removed and the firstinput/output terminal 13 side is the primary side, the timing oftransition to various states may be determined on the basis of theoutput of the second determination circuit 22.

There is a proportional relationship between a load current value of theprimary side of the transformer TR and a load current value of thesecondary side. A current value detected by the current sensor 15 of thesecondary side is the load current value of the secondary side.Accordingly, if the second current sensor 15 and the seconddetermination circuit 22 are removed and the second input/outputterminal 13 side is the primary side, the timing of transition tovarious states may be determined on the basis of the output of thesecond current sensor 15.

It is only necessary for the power conversion device to have a functionof converting a voltage within the first range applied between the firstinput/output terminals 13 into a voltage within the second range andoutputting the voltage from the second input/output terminal 13 and afunction of converting a voltage within the second range applied betweenthe second input/output terminals 13 into a voltage within the firstrange and outputting the voltage from the first input/output terminal13. Accordingly, according to a combination of the first range and thesecond range, a unit 20 having only a function of causing the DC/DCconverter 10 to operate as a step-up (or step-down) converter whosefirst input/output terminal 13 side is the primary side or a step-down(or step-up) converter whose second input/output terminal 13 side is theprimary side may be adopted as the control unit 20.

Also, the procedure of control of the DC/DC converter 10 by the controlunit 20 need not be the same as that described above. For example,although the control unit 20 ascertains a control timing with aso-called software interruption, the control unit 20 that ascertains acontrol timing by a hardware interruption and re-sets the hardwareinterruption when it waits for the change rate to be a value within thechange rate range may be adopted.

What is claimed is:
 1. A power conversion device comprising: a firstinput/output terminal pair; a second input/output terminal pair; a DC/DCconverter connected to the first input/output terminal pair and thesecond input/output terminal pair; and a control unit configured tocontrol the DC/DC converter, wherein the DC/DC converter comprises: afirst switching leg having first and second switching elements connectedin series via a first connection point and connected to the firstinput/output terminal pair; a second switching leg having third andfourth switching elements connected in series via a second connectionpoint and connected in parallel to the first switching leg; a thirdswitching leg having fifth and seventh switching elements connected inseries via a third connection point and connected to the secondinput/output terminal pair; a fourth switching leg having sixth andeighth switching elements connected in series via a fourth connectionpoint and connected in parallel to the third switching leg; a firstenergy storage and conversion unit connected to the first connectionpoint and the second connection point and connected to one winding of atransformer and a first reactor connected in series; and a second energystorage and conversion unit connected to the third connection point andthe fourth connection point and connected to the other winding of thetransformer and a second reactor connected in series, wherein thecontrol unit is able to execute a first control for causing the DC/DCconverter to convert a voltage within a first range applied to the firstinput/output terminal pair into a voltage within a second range and tooutput the voltage within the second range from the second input/outputterminal pair and a second control for causing the DC/DC converter toconvert a voltage within the second range applied to the secondinput/output terminal pair into a voltage within the first range and tooutput the voltage within the first range from the first input/outputterminal pair, wherein the first control is a control for controllingON/OFF of each switching element in the DC/DC converter so that a stateof the DC/DC converter iteratively transitions between, in an order of,a first state in which a current input from the first input/outputterminal pair flows through the first reactor, a second state in which acurrent is able to circulate along a path including the first reactor, athird state in which the current input from the first input/outputterminal pair flows through the first reactor in a direction opposite toa direction in the first state, and a fourth state in which a current isable to circulate along a path including the first reactor while flowingthrough the first reactor in a direction opposite to a direction in thesecond state, and is a control for causing the state of the DC/DCconverter to transition from the second state to the third state after afirst load current value, which is a value of a load current flowingthrough the first reactor, is within a first current value range if thefirst load current value is not within the first current value rangewhen the state of the DC/DC converter is to be transitioned from thesecond state to the third state and causing the state of the DC/DCconverter to transition from the fourth state to the first state afterthe first load current value is within the first current value range ifthe first load current value is not within the first current value rangewhen the state of the DC/DC converter is to be transitioned from thefourth state to the first state, wherein the second control is a controlfor controlling ON/OFF of each switching element in the DC/DC converterso that a state of the DC/DC converter iteratively transitions between,in an order of, a fifth state in which a current input from the secondinput/output terminal pair flows through the second reactor and nocurrent is output from the first input/output terminal pair, a sixthstate in which the current input from the second input/output terminalpair flows through the second reactor and a current is output from thefirst input/output terminal pair, a seventh state in which no current isinput from the second input/output terminal pair and no current isoutput from the first input/output terminal pair, an eighth state inwhich the current input from the second input/output terminal pair flowsthrough the second reactor in a direction opposite to a direction in thefifth state and no current is output from the first input/outputterminal pair, a ninth state in which the current input from the secondinput/output terminal pair flows through the second reactor in adirection opposite to a direction in the sixth state and a current isoutput from the first input/output terminal pair, and a tenth state inwhich no current is input from the second input/output terminal pair andno current is output from the first input/output terminal pair in thisorder and is a control for causing the state of the DC/DC converter totransition from the sixth state to the seventh state after a second loadcurrent value, which is a value of a load current flowing through thesecond reactor, is within a second current value range if the secondload current value is not within the second current value range when thestate of the DC/DC converter is to be transitioned from the sixth stateto the seventh state and causing the state of the DC/DC converter totransition from the ninth state to the tenth state after the second loadcurrent value is within the second current value range if the secondload current value is not within the second current value range when thestate of the DC/DC converter is to be transitioned from the ninth stateto the tenth state, and wherein at least one of a determination ofwhether or not the first load current value in the first control iswithin the first current value range and a determination of whether ornot the second load current value in the second control is within thesecond current value range is performed on the basis of a pattern ofchange in a value of a current flowing through the first reactor or thesecond reactor with respect to time.
 2. The power conversion deviceaccording to claim 1, further comprising: a current sensor for measuringthe value of the current flowing through the first reactor, wherein thecontrol unit determines whether or not the first load current value iswithin the first current value range on the basis of the pattern ofchange in the value of the current flowing through the first reactormeasured by the current sensor with respect to time during the firstcontrol, and wherein the control unit determines whether or not thesecond load current value is within the second current value range usingthe value of the current flowing through the first reactor measured bythe current sensor as the second load current value during the secondcontrol.
 3. The power conversion device according to claim 1, furthercomprising: a current sensor for measuring the value of the currentflowing through the second reactor, wherein the control unit determineswhether or not the second load current value is within the secondcurrent value range on the basis of the pattern of change in the valueof the current flowing through the second reactor measured by thecurrent sensor with respect to time during the second control, andwherein the control unit determines whether or not the first loadcurrent value is within the first current value range using the value ofthe current flowing through the second reactor measured by the currentsensor as the first load current value during the first control.